A process is described in WO 01/51194 (Accentus plc) in which methane is reacted with steam, to generate carbon monoxide and hydrogen in a first catalytic reactor; the resulting gas mixture is then used to perform Fisher-Tropsch synthesis in a second catalytic reactor. The overall result is to convert methane to hydrocarbons of higher molecular weight, which are usually liquid under ambient conditions. The two stages of the process, steam/methane reforming and Fisher-Tropsch synthesis, require different catalysts, and catalytic reactors are described for each stage. The catalytic reactors enable heat to be transferred to or from the reacting gases, respectively, as the reactions are respectively endothermic and exothermic; the heat required for steam/methane reforming is provided by combustion of methane. A potential problem with this process is that other reactions may occur in the steam/methane reformer reactor, either to generate carbon dioxide, or to generate coke. It is suggested that the reformer may incorporate a platinum/rhodium catalyst, the reaction being performed at 800° C. The suggested process relies on a steam/methane ratio that is close to 1:1, as the rhodium catalyst is apparently resistant to coking. An improved way of performing this process has now been found.